The present invention relates to piezoelectric polymers in the form of single or multilayer films. Piezoelectric properties are manifested when these films are placed between two electrodes and an electrical voltage is applied to the latter. Deformations then occur in a direction perpendicular to the faces and in two directions contained in the plane tangential to the faces. Conversely under the action of a mechanical stress a voltage proportional to the stress is produced by a reverse piezoelectric effect. The obtaining of piezoelectric properties devolves from the existence of a permanent dipolar orientation which does not exist when the material crystallizes from the molten state. Thus, at this stage the polymer material is in a non-polar .alpha. phase.
Among the polymer materials which are able to acquire piezoelectric properties reference can be made to polyvinyl chloride PVC, polyvinyl fluoride PVF, polyvinylidene fluoride PVF.sub.2 and its copolymers with polytetraluoroethylene PTFE, polytrifluoroethylene PT.sub.r FE, polyvinyl fluoride PVF and mixtures using polymethyl methacrylate PMMA.
It is known to carry out two successive treatments of these materials, particularly polyvinylidene fluoride PVF.sub.2 by exerting a pulling action thereon in order to obtain irreversible elongations which can reach 300 to 500%, which transforms the initial phase into a polarizable phase and said phase is then oriented under the action of a strong electric field close to the dielectric strength limit of the film.
The conventional production operations are generally carried out successively and independently. They consist of the formation by melting of granular materials and the cooling under a press of a plate-like blank, the mechanical stretching of the blank between the jaws of a pulling machine and polarization of the stretched film by applying a voltage between electrodes deposited on the film or by corona discharge.
In connection with the stretching of the film a continuous treatment operation is possible, which consists of winding the film from a supply reel on to a take-up reel, which rotates at a higher speed. Bearing in mind the forces involved it soon becomes clear that this method can only be used for thin films whose thickness does not exceed 50 microns and provided that the said films are heated to temperatures equal to or above the thermoplasticity threshold.
With regards to the continuous polarization it is possible to pass the previously stretched film between a supply reel and a take-up reel revolving at the same speed. The application of a corona discharge to the unwound film makes it unnecessary to apply a metal coating to at least one of the faces of the film. Metallization of the stretched film can also be obviated by passing it between two fixed electrodes or two conductive rollers. However, none of these polarization processes is completely satisfactory, because the electric field necessary for obtaining satisfactory piezoelectric coefficients are too close to the breakdown fields. It is necessary to select an adequate polarization temperature to enable the bipolar orientation mobility to take place, which leads to a temperature of at least 80.degree. C., in the case of polyvinylidene fluoride. At this temperature the ionic conductivity lowers the dielectric breakdown threshold in such a way that the electric field which can be applied in practice is the result of a compromise. Unless a complete dipolar orientation takes place the piezoelectric coefficients obtained in a reproducible manner are for d.sub.33 below 18 pc N.sup.-1 and by increasing the field with a breakdown risk below 22 pc N.sup.-1. On the basis of what has been stated hereinbefore it is apparent that there is a considerable need for a continuous production process having a reproducibility level close to unity making it possible to treat films which are as thick as 1 mm and which produces oriented films, whose piezoelectric coefficients exceed the hitherto obtained values.